The storm will arrive from the south late Wednesday morning with an initial outburst of snow, according to the National Weather Service. In its morning discussion, forecasters were predicting accumulation of 2-3 inches (the city) to 4 inches (Chester County). In issuing the winter storm watch, however, they nudged accumulations to 3 to 5 inches of snow, with higher amounts in the western suburbs.

Accu-Weather sees accumulation totals a bit higher, with 3 inches in the city and as much as 6 inches of snow in some places west of the city.

Sleet and freezing rain will gradually mix in with the snow. With the latest update, it appears the region will now see a prolonged period of freezing rain – not the all-rain in earlier forecasts – on Wednesday night and perhaps into Thursday morning, the weather service said. A glazing of ice is possible.

The weather service noted that there is much uncertainty right now about expected snow and ice accumulations, noting the speed of transition to a mix and then rain could have significant impact on accumulations. Its forecast represents a compromise between one model, the Global Forecast System, or GFS, which shows just 2 inches of snow, and another model, the North American Mesoscale Forecast System, or NAM, which shows a half-foot of snow in Philadelphia, forecasters said in the afternoon weather discussion.

Source/NOAA / National Weather Service

One thing seems for sure: travel is expected to be difficult during the storm, with low visibility and slippery snow-covered streets, and forecasters urged extreme caution. The wintry mix will perpetuate slick conditions on cleared roads and sidewalks.

The storm could lead to delays and cancellations at Philadelphia International Airport, where deicing operations and low visibility conditions can be expected, AccuWeather cautioned, noting that other Mid-Atlantic hubs such as New York, Washington and Boston will see similar problems.

According to Accu-Weather, the "massive" storm will affect 200 million Americans in 39 states beginning Tuesday. Heavy rain and flooding is forecast in southern states.

Here is the 7-day forecast for Philadelphia from the weather service:

Monday: Mostly sunny, with a high near 47. Breezy, with a west wind 15 to 21 mph, with gusts as high as 31 mph.